1. Field of the Invention
The present invention is directed to the design and manufacture of integrated circuits. More specifically, but without limitation thereto, the present invention is directed to the design and testing of logic for detecting transition delay faults in an integrated circuit.
2. Description of Related Art
To facilitate testing of integrated circuits using automated test equipment (ATE), test logic is generally included in the integrated circuit itself. The test logic usually functions only during testing and has no utility during normal operation of the integrated circuit in the field.
In a scan test, the flip-flop elements in the integrated circuit design are connected in large scan chains. During the scan test, test patterns typically generated by an automatic test pattern generator (ATPG) are shifted through the scan chains to initialize the flip-flop elements to a predetermined state. The output data from the logic elements connected to the flip-flop elements is captured by other flip-flop elements that are also part of the scan chains. The captured data is then shifted out. The automated test equipment (ATE) controls the shifting and capture phases and transmits the test patterns to the integrated circuit. The same clock signal is used during the shift and capture phases, and a scan enable signal is used to control when shifting occurs and when data is captured. The ATE also receives the captured data when it is shifted out and compares the captured data to expected results. There are two types of transition delay fault (TDF)/inline resistance fault (IRF) pattern formats: double-capture and launch-off-shift. The double-capture format generates two capture clock pulses when the scan enable signal is driven LOW, while the launch-off-shift format generates only one capture clock pulse when the scan enable is driven LOW. The launch-off-shift method of testing is called launch-off-shift because the logic under test begins to transition to the state that is captured immediately after the final shift.